The present invention relates to a process and an apparatus for producing synthesis gas.
Following the progress made in the development of fuel cells for generating electrical energy, there exists an increased demand for hydrogen-containing fuels for the fuel cell reaction. These fuels may be either hydrogen gas or also a hydrogen-containing compound, e.g. methyl alcohol, which is fed to the fuel cell directly or after passing through a gas reforming process. Especially when using fuel cells for electrically operated motor vehicles it is advantageous when the fuel can be made available directly inside the vehicle. A possible process for producing hydrogen as fuel or as starting product for producing a hydrogen-containing fuel is, for example, the known production of synthesis gas from the substances CO2 and H2O. At present the synthesis gas production takes place by high-pressure electrolysis processes at approx. 1000xc2x0 C. on large plants.
The principle of such a synthesis gas production is described, for example, in the article xe2x80x9cProduction of liquid fuels from atmospheric carbon dioxidexe2x80x9d by M. Specht and A. Bandi, published in xe2x80x9cThemen 94/95xe2x80x9d of the Forschungsverbund Sonnenenergie (Solar Enemy Research Institute). According to this publication a tube reactor is used for the synthesis gas production. However, for use in motor vehicles a process is required which permits, in the smallest possible space, a highly efficient yield of synthesis gas.
It is, therefore, the object of the present invention to indicate a process and an apparatus which permits the production of synthesis gas in the smallest possible space, e.g. on board a motor vehicle.
According to the invention this object is addressed by a process for producing synthesis gas by electrolysis, characterised in that the electrolysis is carried out using textile micro-hollow fibers as solid electrolytes, the inside and outside surfaces of which carry the anodes and cathodes respectively, as well as by an apparatus for producing synthesis gas by electrolysis, characterised in that it comprises a multitude of stacked textile micro-hollow fibers as solid electrolytes, the inside and outside surfaces of which carry the anodes and cathodes respectively, wherein the ends of the micro-hollow fibers are bound in a form-stable manner into a frame.
The use of textile micro-hollow fibers as solid electrolytes has the advantage that for the electrolysis reaction only little space is required at a simultaneously large electrode surface. Understood under textile fibers are in general those fibers that can be processed in the manner of textiles. Such fibers have a great length compared to their cross-section as well as sufficient strength and flexibility. Because of the small diameter and the corresponding small wall thicknesses of these fibers, the diffusion through the hollow fibers takes place in an extremely short time compared to the tube reactors of the state of the art. For the use of micro-hollow fibers in the process according to the invention, they must be extremely homogeneous with regard to their dimensions, so as to ensure a uniform reaction pattern. For this reason the fluctuations of wall thickness and outside diameter of the micro-hollow fibers should not be more than each approx. +/xe2x88x9210%, in particular not more than each approx. +/xe2x88x928%. Particularly preferred are those micro-hollow fibers with which these fluctuations each lie under +/xe2x88x926%.
The manufacture of such micro-hollow fibers is described, for example, in the EP-A-0 874 788 of the same applicant. The micro-hollow fibers can be made with very small wall thicknesses of approx. 0,01 to 15 xcexcm and outside diameters from down to 0,5 to 35 xcexcm. Because of the small dimensions such micro-hollow fibers display textile properties, i.e. they can be bent particularly easily without breaking. By the manufacturing process described in the EP-A-0 874 788 the micro-hollow fibers can be manufactured with extremely accurate dimensions, wherein the fluctuation range of wall thickness and outside diameter is not more than approx. +/xe2x88x926%. The precision of maintaining the values of the diameter and in particular of the wall thickness ensures a homogeneous reaction pattern over the entire length of the hollow fibers.
Alternatively, it is possible to manufacture the required hollow fibers from flat, smooth or structured, plastic bipolar electrolytic foils, which are rolled into straws or wound into helical or spiral tubes. In this way, in particular hollow fibers with an outside diameter of approx. 0,28 to 10 mm can be produced. When rolling the foils into straws, one proceeds in this connection in the same way as with the process which is known, for example, for making cigarettes. The lengths of the straws or spiral tubes produced in this manner generally lies preferably between approx. 0,03 m and 3,00 m, in which connection for the use of the straws in the process according to the invention for the synthesis gas production a length of approx. 0,03 m to approx. 0,20 m is preferred. It is possible to realise any desired and technically expedient length/diameter ratio. After the forming of the straws or spiral tubes, they are ceramically baked. The electrolytic foils can be extruded together with the electrode material before they are processed further into straws or spiral tubes. To manufacture the electrolytic foils, in this connection in particular the sol-gel process can be used.
By using structured, e.g. pleated, or curved or corrugated electrolytic foils for making the straws or spiral tubes, the surface of the straws or tubes that is available for the reaction can be increased further. Another advantage of the use of structured foils lies in the increased bending strength of the straws and tubes produced therefrom.
As starting materials for the micro-hollow fibers used in the process according to the invention, among others hydrolysable substances, collagens, proteins, urea, gelatine, starches or ceramic materials or the precursors of a ceramic material can be used.
The thermal energy required for the synthesis gas production can be made available, for example, by wetting a zeolite. Also suitable for storing heat are melts of aluminium and tin. Also the use of a catalytic atmospheric burner is conceivable.
To improve the reactivity and to achieve a lower operating temperature, the textile micro-hollow fibers are coated on both sides with a catalyst, preferably an element of the eighth sub-group, wherein platinum is regarded as particularly suitable. With this catalyst an effective synthesis gas reaction can take place already at approx. 85xc2x0 C. to 180xc2x0 C. or 500xc2x0 C. to 730xc2x0 C. in the case of the high-temperature electrolysis and at a pressure of approx. 0,1 to 3 bar. The pressure vessel must in this case be designed for a pressure up to 12 bar. When using a platinum catalyst, the number of atoms per cluster is preferably increased to 8 to 15. In this way an additional saving of material can be obtained. Furthermore, because of the increased number of atoms in the clusters, the starting time of the catalyst can be shortened and the reactivity of the reactants increased. The catalyst must be porous so that it will not prevent the flow of gas through the micro-hollow fiber membrane. Also suitable as catalysts are the following materials: spinnel (MgAlO), titanium dioxide (in its anatase form), vanadium pentoxide, tungsten trioxide, copper oxide, molybdenum oxide and iron oxide. Heavy as well as light platinum is suitable as catalyst material.
Preferably, the textile micro-hollow fibers are made from an oxide of the titanium group which is stabilised with a rare earth metal. Particularly preferred for the process according to the invention are textile micro-hollow fibers which are made from yttrium-stabilised zirconium oxide. This material is known as electrolyte material and has proved eminently suitable under the temperature conditions required for the electrolysis.
For use in the process according to the invention for the synthesis gas production, micro-hollow fibers with an outside diameter of approx. 1 xcexcm to 10 mm, especially between 50 xcexcm and 280 xcexcm, are particularly preferred. Hollow fibers with these dimensions can easily be arranged in stacks and are easier to handle. In addition they offer an excellent surface/volume ratio.
The process according to the invention is preferably carried out in such a way that carbon dioxide and water or water vapour are each fed into one end of the hollow fiber lumen and are subjected to a high-temperature electrolysis, wherein the oxygen passing out through the wall of the hollow fibers as permeate is drawn off separately from the carbon monoxide and hydrogen synthesis gas flowing out of the second end of the hollow fiber lumen. The occurring oxygen can then, for example, be fed to a fuel cell. In this way a utilisation of the hydrogen gas contained in the synthesis gas as well as of the xe2x80x9cby-productxe2x80x9d oxygen is possible.
To separate the hydrogen contained in the synthesis gas from the carbon monoxide after the flowing out of the lumen of the hollow fiber electrolyte, the hydrogen can be passed through a gas separating membrane. The pororosity of the membrane must then be chosen in such a way that the gaseous hydrogen can diffuse through it, whereas the carbon monoxide remains on the inlet side. An as clean as possible separating of the two gas constituents is particularly desirable from the point of view of a high efficiency of the subsequent process steps, during which one or both constituents of the synthesis gas are used.
The gas separating membrane may also be in the form of micro-hollow fibers. In this case the synthesis gas flowing out of the electrolyte micro-hollow fibers is fed into the lumen of the gas-separating hollow fibers. The dimensions of the micro-hollow fibers used for the gas separation can be substantially the same as those for the synthesis gas production, wherein on the hollow fibers for the gas separation no electrodes are required.
The process according to the invention can be used within the framework of a system for generating electrical energy by fuel cells, in particular on board a motor vehicle. With this the synthesis gas reaction supplies the hydrogen gas required for the operation of the fuel cell, which can be fed directly to the fuel cell, or can be processed further into hydrocarbon-containing compounds, in particular methyl alcohol. The intermediate step of the methyl alcohol production has the advantage that the produced methyl alcohol, because it is liquid under normal conditions, can easily be stored or put into a buffer store, e.g. on board a motor vehicle. In this way the production of the fuel and the use thereof can be better separated with regard to time.
The hydrogen contained in the synthesis gas can, therefore, after its separation be fed to a fuel cell as fuel. In the fuel cell an oxidation reaction then takes place with the release of energy which can be used, for example, to drive an electric vehicle.
In terms of a multiple utilisation of the reaction products in question, preferably at least one reaction product of the fuel cell reaction is fed back into the synthesis gas reaction. For example, the carbon dioxide formed during the fuel cell reaction can also be used for the methyl alcohol synthesis according to CO2+3H2xe2x86x92CH3OH+H2O.
The carbon monoxide contained in the synthesis gas can be separated and with hydrogen or a hydrogen-supplier can be synthesised into a hydrocarbon-containing compound, in particular methyl alcohol. The relevant reaction CO+2H2xe2x86x92CH3OH is preferred for the synthesis of methyl alcohol over the methyl alcohol production according to the equation CO2+3H2xe2x86x92 CH3OH+H2O, as the former reaction on the one hand takes place without producing product water and on the other hand, during the second reaction the catalysts quickly display ageing symptoms, which require frequent material renewals.
The hydrogen required for the methyl alcohol synthesis is preferably obtained from salt solutions of elements of the eighth sub-group, in particular palladium rhodium, ruthenium, osmium, iridium or platinum, wherein as catalysts metal hydrides, in particular magnesium hydride, and/or metal carbides are used and the required reaction energy is obtained by means of an electrolyser, e.g. a wind-driven electrolyser. The reaction itself, which occurs for example according to the equation Pd+H2Oxe2x86x92Pd*2H2+CO2, can take place in micro-hollow fibers that have the same dimensions as those used in the synthesis gas production. Also in this case the hollow fiber walls are made as electrodes, the inside wall acting as anode. The hydrogen is then released inside the micro-hollow fibers and can, for example, be separated by means of a molecular sieve, whereas the carbon dioxide flows out through the micro-hollow fiber wall. This hydrogen source can be used, besides for the synthesis gas production according to the invention, for the methyl alcohol production.
Hydrogen can furthermore be obtained from reactions according to the equation C+2H2Oxe2x86x92CO2+2H2 or CO+H2Oxe2x86x92CO2+H2. In addition to carbon monoxide and its compounds, it is also possible to obtain the required hydrogen on the basis of silicon or its compounds. Here especially the reactions Si+3H2Oxe2x86x92H2SiO3+2H2 or SiH4+2H2Oxe2x86x92SiO2+4H2 can be mentioned. For the subsequent reduction of the SiO2, among others magnesium or aluminium can be used. These are endothermic reactions that take place according to the equations 2Mg+SiO2xe2x86x92Si+2MgO or 4Al+3SiO2xe2x86x923Si+2Al2O3. The reaction product Al2O3 can be removed from the reaction cycle by cyclone separators and be used again, e.g. as raw material for the manufacture of micro-hollow fibers.
Finally, also carbides, especially magnesium and calcium carbide, can be used as hydrogen carriers within the framework of the reactions Mg2C3+4H2Oxe2x86x922Mg(OH)2+C3H4 or CaC2+2H2Oxe2x86x92Ca(OH)2+C2H2. In addition, carbides in a fuel cycle are also suitable as heating media for making available the thermal energy required for the endothermic reactions.
The carbon dioxide required for the synthesis gas reaction can be obtained, for example, from reactions of carbon-containing compounds with water or water vapour, from gases that contain carbon dioxide or from solid storage media, in particular from barium carbonate. It is possible, in particular, to use carbon dioxide from the air. As solid storage medium barium oxide is suitable, for example, in which case the carbon dioxide is stored according to the equation BaO+CO2xe2x86x92BaCO3. Also a deposition of the carbon dioxide in a molecular sieve is conceivable. In addition to the synthesis gas production, the carbon dioxide can also be converted according to the equation CO2+H2xe2x86x92CO+H2O or can be used for producing methane according to CO2+4H2xe2x86x92CH4+2H2O, which in turn constitutes a storage medium for hydrogen for the fuel cell operation.
Preferably, the heat exchange between the reactants and the reaction products of at least one of the aforementioned reactions is carried out by using micro-hollow fibers with non-activated surfaces as heat exchangers, which are operated in particular in the cross-current mode. This is advantageous, for example, with the synthesis gas production according to the invention itself. In this case the carbon dioxide and the water or water vapour, before being fed into the electrolyte micro-hollow fibers, are fed jointly into the inside of micro-hollow fibers with non-activated surfaces, where they are heated by the reaction products of the synthesis gas production flowing perpendicular to the longitudinal axis of the heat exchanger fibers. In this way the energy consumption required for the high-temperature electrolysis can be reduced considerably.
The produced methyl alcohol can either be used directly as fuel in a so-called DM fuel cell (=Direct Methanol fuel cell), or it can be subjected to a so-called gas reforming process according to the reaction equation CH3OH+H2xe2x86x92CO+3H2, in which case the released hydrogen can be used, for example, for the operation of a conventional fuel cell (Proton Exchange Membrane Fuel Cell). The advantage of using the hydrogen obtained from the reforming process over the direct use of methyl alcohol in a DM fuel cell lies in the considerably greater power yield of the first mentioned method. Thus, when using hydrogen as fuel, an efficiency of 60% can be obtained compared to an efficiency of only approx. 20% for the direct use of methyl alcohol.
The process of the gas reforming can also be carried out using micro-hollow fibers as reaction vessels, wherein the micro-hollow fibers used for this process preferably are made from the same starting material as the micro-hollow fibers used for the synthesis gas production and are also coated with catalysts, preferably from elements of the eighth sub-group. The thermal energy required for the reforming process can again be made available by an electrolyser or by means of microwaves. Also here the use of micro-hollow fiber heat exchangers can produce a high energy saving.
The apparatus suitable for producing synthesis gas by electrolysis comprises a multitude of stacked textile micro-hollow fibers as solid electrolytes, the inside and outside surfaces of which carry the anodes and cathodes respectively, wherein the ends of the micro-hollow fibers are bound in a form-stable manner into a frame. The stacked micro-hollow fibers accordingly form a disc of finite thickness, which is delimited by the frame. The binding of the micro-hollow fibers into the frame can take place in any suitable manner, e.g. by casting the hollow fiber ends into the frame. At the outer periphery of the frame the ends of the hollow fibers are exposed so that access to the hollow fiber lumen is ensured.
Preferably, the micro-hollow fibers inside the stack are arranged parallel to one another, in which case the frame has a rectangular or square shape. The micro-hollow fibers of a stack accordingly all have substantially the same length. With this embodiment the feeding of the materials into the hollow fiber lumen takes place on two opposite sides.
The apparatus according to the invention may have at least one further stack of textile micro-hollow fibers which act as heat exchangers between the reactants and the reaction products of the synthesis gas production. The advantage of using heat exchangers for the synthesis gas production has already been described in the foregoing.
According to a particularly preferred embodiment the apparatus according to the invention has at least one further stack of textile micro-hollow fibers which act as gas separating membrane for the reaction products of the synthesis gas production.
The individual stacks preferably have the same size and shape, so that as function modules they can in turn be stacked on top of one another and/or be arranged offset by 90xc2x0 relative to one another. In this way multi-function modules can be produced, the dimensions of which are so small that they can easily be accommodated on board an electric motor vehicle to supply fuel for a fuel cell which serves to produce the drive energy for the vehicle. The energy required for the endothermy of the described reactions can be made available by a so-called electrolyser, e.g. a wind-driven electrolyser.
The individual stacks or function modules may be accommodated according to the invention in a pressure housing which is made from a ferromagnetic material and the surface of which is partly enamelled. The use of partly enamelled pressure vessels for the processes of the high-temperature electrolysis is known in the state of the art. The enamel layer serves in this case to avoid contact between highly heated reactants and the ferromagnetic material.
The non-enamelled part of the surface of the pressure housing may be provided with a first coating of aluminium nitride. Aluminium nitride, which has excellent heat conduction properties, serves as scale protection for the non-enamelled surface parts. During the manufacture of the housing a layer of carbon or graphite placed on the part of the surfaces coated with aluminium nitride protects same against oxidation reactions during the enamelling and welding operations. In this way a stability of the material also at temperatures up to over 1100xc2x0 C. can be achieved.
According to an alternative embodiment, the non-enamelled part of the surface of the pressure housing may be provided with a layer of gold. Gold has the advantage of a very high heat conductivity as well as indifference to oxidation processes, which is why in this case a carbon or graphite layer during the welding and enamelling processes can be dispensed with and the gold plating can be provided directly on the blank steel. The chemo-selective gold plating replaces here the previously used silver plating. As a result of the gold plating the operating surfaces become more chemically resistant. Notwithstanding the comparatively high price of gold, in this way corrosion of the surfaces of the non-enamelled part of the pressure housing can be avoided in a relatively economical manner, seeing that the gold plating need only be applied in a thickness in the xcexcm-range.